The light generated in XFELs driven by the plasma-based accelerators will have unique properties very different from those of XFELs driven by conventional RF linacs. In the baseline scenario, the light pulse will have a small number of photons and a large chirp. This kind of light is not suitable for imaging [1]} and coherent spectroscopy [2]} experiments, which require monochromatic light pulses. At the same time, unique properties of the light may be utilized for designing different class of experiments. The light pulse with a strong chirp opens possibility for its further compression below the attosecond duration similar to the chirped pulse amplification (CPA) technique for visible light [3]}. Strong chirp also allows for time resolved measurements with subfemtosecond resolution since wavelength sensitive streaking is equivalent to time streaking [4]}.
